Skip the auto shop - Our mechanics make house calls

P0109 OBD-II Trouble Code: Manifold Air Pressure (MAP) Barometric Pressure Sensor Electrical Circuit Intermittent Malfunction

Check Engine Light

P0109 Trouble code definition

Manifold Air Pressure (MAP) Barometric Pressure Sensor Electrical Circuit Intermittent Malfunction.

What the P0109 code means

P0109 is the code for a problem with the MAP circuit sensor having intermittent voltage input signals to the Engine Control Unit (ECU). This means the voltage input to the ECU is too erratic, which means that it is not in the correct range, or it changes to on and off preventing proper engine operation to work with inputs from the Mass Air Flow Sensor (MAF) and Throttle Position Switch (TPS).

What causes the P0109 code?

The MAP circuit voltage input to the ECU being too erratic may have several causes:

  • The source of the problem is the sensor sending improper voltage information to the ECU.

  • The most common problem is a bad MAP sensor.

  • The wiring or connector may be bad or have a bad connection or wiring. It could be too close to higher voltage consumption components, especially alternators or ignition wires causing fluctuating voltages. A poor electrical ground can cause problems also.

  • The sensor itself may be simply operating out of range from age, fatigue, or damaged parts of the sensor components internally.

  • MAP sensors must operate within specific ranges to send correct signals to the ECU. This ensures proper engine operation and smooth power output

What are the symptoms of the P0109 code?

P0109 code will be usually preceded by the Check Engine Light coming on the vehicle dashboard display. The vehicle will most often not run well, idle poorly, accelerate erratically, run rich and backfire because the MAP sensor and throttle position sensor are not operating together.

How does a mechanic diagnose the P0109 code?

P0109 is diagnosed with an OBD-II scanner compatible with the vehicle. A qualified technician should then reset the OBD-II fault codes and road test the vehicle to see if the code and or Check Engine Light comes back on. He can observe this by watching it live on his scanner while driving.

If the code comes back, then the mechanic will need to do a voltage test with the key on using a multimeter and verifying there is 5 volts input to the MAP sensor and a minimum of .5 to 1 volt with throttle closed. With the engine running at idle, it should have minimum 1 volt on the input to the ECU and then increase smoothly with engine speed and load. the voltages should not fluctuate at idle or steady engine speed. If the MAP Sensor input voltages to the ECU fluctuate erratically, then it is most likely the MAP sensor is bad and needs replacement. The technician must check the wiring and connector to make sure it is not simply a loose or weak connector causing the fluctuations.

Common mistakes when diagnosing the P0109 code.

Diagnostic errors are largely due to not following the code’s diagnostic procedure. First, follow the test procedure in the diagnosis to ensure there is the correct voltage to the sensor and from the sensor to the ECU. The technician must verify the voltage output of the MAP Sensor is in the correct range and smoothly fluctuates with the engine speed and has proper voltage. Idle voltage is normally 1 to 1.5 volts and full throttle is usually around 4.5 volts.

Do not buy a new MAP Sensor or ECU unless it is clearly at fault.

How serious is the P0109 code?

The P0109 code will result in very poor running of the engine and requires immediate attention. It is necessary for technician to check it out as soon as possible. The MAP sensor issue can cause excessive fuel consumption, rough operation, backfire, ignition miss and difficulty starting in certain circumstances, and can cause other damage including the engine internally if continued to be driven. Occasionally, if no problems are found, reset the fault codes and then retest.

Often times, if the Check Engine Light came on immediately at start up, then the OBD- II system can be reset and the vehicle will operate normally.

What repairs can fix the P0109 code?

  • Verify the code with a scanner. Reset the fault codes and perform a road test to see if the Check Engine Light comes back.

  • If the Check Engine Light comes back, check the MAP sensor voltage, which is normally 5 volts in and 1 to 4.5 volts out to the ECU along with the electrical connector and wiring. Disconnect the electrical connector and then reinstall to ensure a fresh electrical connection. Then check the voltage output on the MAP sensor to see if it is in the correct range and not changing erratically. Check the grounds to ensure they are secure and clean.

  • At this point, if the readings and wiring are normal, it is best to to determine if the MAP sensor is defective and if it has incorrect output, then replace the MAP sensor. If all checks are good, then a final test to determine if the ECU is bad must be done.

Additional comments for consideration regarding the P0109 code

Many vehicles with mileage over 100,000 have momentary sensor problems that usually occur during start up or prolonged stress situations on the drive train. If the Check Engine Light comes on and the vehicle seems to be operating normally, the OBD-II system can be reset using the scanner and the problem may not reoccur. This is why it is important to verify the fault and reset it before doing any repairs.

Need help with a P0109 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.


Post a question and get free advice from our certified mechanics.


More related articles

P0121 OBD-II Trouble Code: TPS "A" Circuit Range Performance Problem
P0121 code definition Throttle Pedal Position Sensor/Switch (TPS) A Circuit Range Performance Problem What the P0121 code means P0121 is a generic OBD-II code which indicates that the Engine Control...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
Veteran and Military Driver Laws and Benefits in Idaho
The state of Idaho offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch in the...

Related questions

Q: What is a throttle body filter

Code P0068 can have to do with the Throttle Position Sensor. It can also be due to a Mass Air Flow sensor fault. Usually this code can be set due to a vacuum leak, either somewhere in the air intake...

Q: Truck was stolen and ditched now it won't start

If the truck was stolen there could be a number of things causing it to not start now including several anti-theft devices on the truck. I would recommend having an expert, like from YourMechanic, come to your home to diagnose...

Q: What is the best fix for a catalyst system efficiency below threshold (bank 2)

Trouble code P0430 is for a catalyst on the drivers side. The sensor after the converter may be bad and not responsive enough or the catalyst itself may be bad. The catalyst should be replaced if the converter rattles, there...